1. Field of the Invention
The present invention relates to a utility vehicle, in particular a fire fighting vehicle, comprising an extendable areal apparatus like a turnable ladder and/or an areal rescue platform and a jacking system.
2. Description of the Related Art
When the areal apparatus of a utility vehicle, such as a fire fighting vehicle, is extended, it is important to secure the vehicle against a tilting movement in the lateral direction. For this purpose, jacking systems are provided that comprise lateral ground supports for jacking a vehicle body. These supports can be extracted from the vehicle body into an operating position in which the ends of the supports rest on the ground, so that the weight load of the vehicle rests on the supports, and the vehicle axles and their suspensions are relieved. In some cases the vehicle is lifted from the ground completely, so that the wheels do not have contact with the ground anymore. However, in other cases it is completely sufficient to lift the vehicle body from the axle in a way that the wheels keep a minimum of ground contact.
To perform this deflection of the vehicle load to the lateral ground supports, the jacking system comprises stabilization means with a rear axle blocking device for blocking the rear axle suspension of the vehicle. This rear axle blocking device can be operated hydraulically and may comprise, for example, a pulling means for pulling the rear axle in an upward direction against the vehicle body, i.e. against the spring force of the rear axle suspension. In its activated state, with the rear axle pulled against the vehicle body, the rear axle suspension is blocked completely to prevent any relative movement of the axle against the vehicle body. In a simple and common construction, the pulling operation of the rear axle blocking device is performed by a steel rope connecting the rear axle with the end of a hydraulic cylinder and being guided over a deflection pulley. If the hydraulic cylinder is retracted, the rear axle is pulled in an upward direction, as described above.
However, even if the rear axle is blocked, the stabilization means cannot prevent a tilting movement of the vehicle body completely in some operation states of the areal apparatus. This is due to the arrangement of the lateral ground supports at the length position of the vehicle. While the rear pair of ground supports can be arranged at the rear end of the vehicle, the front pair of ground supports is usually arranged between the front and rear axles. When the areal apparatus is extended in the front direction (i.e. the driving direction) of the vehicle, extending over the driver's cabin, the center of gravity of the vehicle shifts in this direction. Consequently the load of the vehicle is deflected towards its front axle, and the vehicle tends to perform a tilting movement around the front pair of ground supports, slightly lifting the rear end of the vehicle and lowering the driver's cabin against a spring force of the front suspension of the front axle. In another operating state of the areal apparatus, when it is extended in the opposite direction over the rear end of the vehicle, the spring force of the front axle suspension tends to promote a lifting of the driver's cabin, so that a tilting movement of the vehicle in the opposite direction over a rear pair of ground supports is promoted.
It is clear from the above that the usual spring dampening function of the front axle suspension promotes tilting movements in such operating conditions of the areal apparatus in an undesired way. However, there is no known way to deactivate the front suspension system. In particular it is impossible to block the front axle in the same way as with the hydraulically operable rear axle blocking device described above, because of the limited construction space in the front axle area and the increased complexity of the construction of the front axle, for example, by the limitations of the steering system. It must also be considered that a complete unloading of the front axle by pulling up the axle against the vehicle body, as with the rear axle blocking device, would even promote the front tilting of the vehicle when the areal apparatus is extended over the driver's cabin, so that the desired effect cannot be achieved.